Marked improvements in the performance of photographic emulsions began in the 1980's, resulting from the introduction of tabular grain emulsions into photographic products. A wide range of photographic advantages have been provided by tabular grain emulsions, such as improved speed-granularity relationships, increased covering power (both on an absolute basis and as a function of binder hardening), more rapid developability, increased thermal stability, increased separation of native and spectral sensitization imparted imaging speeds, and improved image sharpness in both mono- and multi-emulsion layer formats.
Although tabular grain emulsions can be selected to provide a variety of performance advantages, depending upon the photographic application to be served, initially commercial interest focused on achieving the highest attainable photographic speeds with minimal attendant granularity, resulting in the commercial development of silver iodobromide {111} tabular grain emulsions.
More recently interest has developed in the higher rates of processing and greater ecological compatibility of high chloride emulsions. The first high chloride tabular grain emulsions contained {111} tabular grains, as illustrated by Wey U.S. Pat. No. 4,399,215 and Maskasky U.S. Pat. No. 4,400,463. Although numerous high chloride {111} tabular grain emulsions have been subsequently investigated, their commercial development has lagged, which is attributable to the tendency of high chloride {111} tabular grains to revert to nontabular forms--i.e., morphological instability. This reversion tendency of high chloride {111} tabular grains is overcome by adsorbing a grain growth modifier to the grain surfaces during preparation. Unfortunately, the grain growth modifier complicates post-precipitation preparation of the grains for imaging, particularly chemical and spectral sensitization.
Maskasky U.S. Pat. Nos. 5,292,632 and 5,275,930 overcame the problem of high chloride {111} tabular grain morphological instability by providing the first high chloride {100} tabular grain emulsions. Unfortunately, Maskasky also relied upon grain growth modifiers to attain high chloride {100} tabular grain emulsions.
House et al U.S. Pat. No. 5,320,938 and Chang et al U.S. Pat. No. 5,413,904 were able to produce high chloride {100} tabular grain emulsions by introducing small amounts of iodide at or near grain nucleation. While this approach has the advantage of avoiding the use of adsorbed grain growth modifiers, it has been observed that the much lower solubility of silver iodide than that of silver chloride, a difference of about 6 orders of magnitude, requires careful control of the process to achieve batch-to-batch replication of precipitations. For example, at 40.degree. C., the -log K.sub.sp of AgCl is 9.2 whereas the -log K.sub.sp of AgI 15.2, where K.sub.sp is the solubility product constant. In other words AgI is one million times less soluble than AgCl.
Still another approach to preparing high chloride {100} tabular grain emulsions begins by precipitating AgCl to create grain nuclei. This precipitation is interrupted to introduce a small quantity of silver and bromide salts that form halide laminae on the grain nuclei. Silver AgCl precipitation is then resumed to form high chloride {100} tabular grains. The introduction of silver bromide after grain nucleation is said to create a halide gap that is responsible for tabular grain growth. Yamashita et al U.S. Pat. No. 5,498,511 most fully describes the halide gap concept.
The modification of AgCl grain nuclei by AgBr laminae formation followed by return to high chloride precipitation is relatively complicated. The reaction vessel must be adjusted from the conditions for AgCl grain nuclei formation to the conditions favorable for AgBr laminae formation. After a very limited amount AgBr is precipitated, the reaction vessel must be again adjusted to provide conditions for high chloride {100} tabular grain growth.